When roast is done, place on large deep platter; keep warm. Skim fat from liquid in Dutch oven. Fill blender about half full with liquid and vegetables from Dutch oven. Cover (with center part of blender cover removed) and blend at low speed until mixture is smooth; pour into large bowl. Repeat until all of mixture is blended. Return gravy mixture to Dutch oven; heat to boiling.

To serve, pour some gravy over and around roast on platter; arrange squash wedges around roast. Garnish with celery leaves. Serve with remaining gravy.